The input video interface of a digital video tape recorder is required to produce several types of signals representative of the timing of the incoming video data. The production of these signals has to continue even if the input data is interrupted or disappears. Several operations of a digital video tape recorder or its input interface should not be performed if the digital video input signal is not present. The digital video signal may be supplied to the input of a digital video tape recorder in form of parallel data which must be accompanied by a clock. Detecting the presence of that clock is preliminary for detecting whether the incoming digital input video data is present.
The typical design of an input data clock presence detector is based on the principle of a retriggerable one shot multivibrator, which time constant is set to be longer than the period of the clock which triggers it. As long as the clock is present, it keeps the multivibrator in its astable state and the detector output in a state corresponding to the clock presence. Disappearance of the clock allows the astable state to expire and the detector output switches to the state corresponding to the absence of the clock. The multivibrator can be implemented as an analog device or a digital circuit. However, the analog version cannot be used inside of a digital application specific integrated circuit (ASIC), and the digital version requires some other clock (reference clock) operating at a higher rate for clocking the digital one shot.
In accordance with an aspect of the invention, there is provided data clock presence detector circuit suitable for implementation inside of a digital ASIC, in which only reference clocks at a rate lower than the data clock rate are available.